This invention relates generally to control of DC motors, and more particularly to methods and apparatus that are particularly useful for efficiently controlling DC pitch motors in wind turbines.
Recently, wind turbines have received increased attention as an environmentally safe and relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted on a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox or directly coupled to the rotor. The gearbox, when present, steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.
On a pitch controlled wind turbine, an electronic controller is used in conjunction with a blade pitch mechanism to pitch the blades around their respective longitudinal axes to control the power output of the wind turbine. Motors are provided to pitch the blades while the rotor is turning.
Some new pitch implementation systems are required to regenerate continuously. For example, the use of pitch control drive systems in wind turbines with one or more DC links as an intermediate link between source and load requires that the DC link(s) absorb regenerative energy under some conditions. For example, DC link(s) supplied by a diode source may be required to absorb regenerative energy when pitch drive motors are decelerating. The requirement to absorb regenerative energy continuously is new and is not believed to have been addressed by previously known wind turbine pitch system configurations.